Although various industries are extant which employ processes for forming a thin layer or film on a solid substrate, one significant industry in which such processes are widely employed is the production of semiconductor devices. In such production processes, heated substrates, such as planar silicon or gallium arsenide wafers or other suitable such materials, are exposed to gases which react to deposit the desired materials on the surface of the wafer. In typical processes of this nature, the deposited materials form epitaxial films which replicate the crystal lattice structure of the underlying wafer.
These coated wafers are then subjected to well known further processes to form semiconductor devices such as lasers, transistors, etc. For example, in the production of integrated circuits, the layers deposited on the wafer form the active elements of microscopic transistors and other semiconductor devices included in the integrated circuit. The thickness, composition and quality of the deposited layers determine the characteristics of the resulting semiconductor devices. Accordingly, the deposition process must be capable of depositing films of uniform composition and thickness on the front face of each wafer. The requirements for uniformity have become progressively more stringent with the use of larger wafers and with the use of apparatus which deposits coatings on several wafers simultaneously.
In a typical prior art deposition apparatus illustrated in FIG. 1, a wafer 10 is mounted in a wafer carrier 12 which, in turn, is mounted on a susceptor 14. The susceptor 14 may be mounted in a rotating support spindle 16. The susceptor 14, the wafer carrier 12 and the wafer 10 typically are located in an enclosed process reactor 18. A heating assembly 20 symmetrically arranged below susceptor 14 heats the susceptor, which thus results in the heating of wafer carrier 12 and wafer 10 mounted thereon. The use of rotating spindle 16 is intended to enhance the temperature uniformity across the deposition area, as well as the uniformity of the source material gases or vapors flowing over the deposition area.
Conventional wafer carriers, such as wafer carrier 12 shown in FIG. 2, include multiple cylindrical pockets 22 on their upper surface for holding the wafers in place as the wafer carrier is rotated during the deposition process. Typically, the pockets have a diameter which is about 0.020 inches bigger than the diameter of the wafer and a depth which is about 0.002 inches deeper than the thickness of the wafer. These wafer carriers ordinarily also include an annular flange 24 which is used for lifting and transporting the wafer carrier into and out from the reaction chamber. On their bottom surface, the wafer carriers may include an annular wall 26 for locating and holding the wafer carrier concentrically on the susceptor as the assembly is rotated during the deposition process, and for creating a gap 28 between the upper surface of the susceptor and the lower surface of the wafer carrier, which gap eliminates localized heating of the wafer carrier resulting from points of contact between the wafer carrier and the susceptor, and thus enhances the uniform transfer of heat from the susceptor to the wafer carrier.
In deposition processes using conventional wafer carriers, the surface temperature of the wafers is usually cooler than the surface temperature of the wafer carrier as a result of the thermal resistance created by the interface between the wafers and the wafer carrier and the different emissivities of the materials from which the wafer carrier and the wafer are made. Having the surface of the wafer cooler than the surface of the wafer carrier causes drawbacks which diminish the quality of the resultant semiconductor devices. Firstly, during the deposition process, source materials are deposited not only on the exposed surface of the wafers, but also on the exposed surface of the wafer carrier. During subsequent deposition cycles, the higher temperature of the wafer carrier surface can cause the deposited materials to be reevaporated from the wafer carrier surface, resulting in contamination of the materials being deposited on the wafers. Furthermore, the higher temperature of the wafer carrier surface results in a nonuniform temperature on the surface of the wafers, particularly along their outer periphery, such that the layer deposited along this portion of the wafers ordinarily is of poor quality and must be discarded.
There therefore exists a need for a system which will achieve a smaller difference in temperature between the wafers and the wafer carrier surface, as well as a more uniform temperature distribution across the surface of the wafers, such that a more uniform coating can be deposited across the entire surface of each wafer. More particularly, there exists the need for a wafer carrier which will promote the uniform heating of the wafers carried thereon.